Abstract

The reactions of several gas-phase metal cations with small hydrocarbons have been studied using ion beam mass spectrometric techniques. We also present several theoretical studies into the sigma bonding between the first and second row transition metal ions and H and CH_3.
Chapter II discusses the three cations, europium, praseodymium, and gadolinium in an attempt to understand the role of f electrons in the reactivity of gas-phase lanthanide ions. Eu^+ and Pr^+ are found to be unreactive with alkanes while Gd^+ readily activates both C—H and C—C bonds. The unreactive metals have only one non-4f valence electron. Oxidative addition of a C—H bond to these metals requires a strong bond to an f electron. Gd^+, with two non-4f valence electrons need not use the 4f electrons and is seen to be very reactive. This reactivity behavior indicates that the 4f electrons of the lanthanides play little role in alkane activation due to the formation of weak sigma bonds.
In Chapter III and VI, we discuss the reasons for the unreactivity of gas-phase chromium ions. Molybdenum ions which have a very similar electrons structure are found to activate C—H bonds of alkanes. The metal ions are studied from the standpoint of gas-phase reactivity as well as the theoretical description of the bonding in the hydride and dihydride ions. The two metals are found to differ greatly in the strength of the sigma bonds that they form to hydrogen. The oxidative addition of C—H and C—C bonds to Cr^+ is endothermic due to the extremely weak bonds formed to the metal ion.
Chapters IV and V report systematic, ab initio, generalized valence bond and configuration interaction calculations on the first and second row transition metal hydrides. The bonding in these systems is seen to depend on a number of factors including: (1) the electronic structure of the metal ions; (2) the sizes of the metal s and d orbitals and the effect on the intrinsic strength of the metal—hydrogen bond; and (3) the mediation of the intrinsic bond strengths by the loss of high spin exchange energy.
Chapter VII presents a theoretical comparison between the metal hydride ions and metal methyl ions. The present theoretical study indicates that for a variety of metal systems, the metal—hydrogen and metal—carbon bonds are very similar, both from the standpoint of metal orbital hybridization as well as bond dissociation energy.